We have carried out in situ measurements of optical anisotropy in the depletion layer of Cl-doped n-type ZnSe during molecular beam epitaxy. Reflectance difference (RD) spectroscopy was employed to measure the anisotropy which was induced by the linear electro-optic (LEO) effect. A power-law relation was obtained between the LEO signature near the E-1 and E-1 + Delta(1) transition energy and the carrier concentration determined from ex situ capacitance-voltage measurements. Using this result we are now able to measure and control the carrier concentration in situ and in real time during ZnSe growth. Quantitative relation between built-in electric field in the depletion layer and the amplitude of the LEO signature of the RD spectra is discussed.